Remove format-util dependency

[JamesMessinger]

Now that JavaScript supports template strings, it's usually not necessary to use a string-formatting function.

Drop the format-util dependency, but continue to support ono.formatter. On Node.js, ono.formatter will default to util.format. In web browsers, it will default to undefined, but users can set it to format-util or their own implementation if they wish.

[JamesMessinger]

We just released Ono v5.0, which removes this dependency as described above.

Here are the release notes